This invention relates to groundspeed measurment systems, using the Doppler shift in the radio frequency of a signal transmitted by a fixed ground station and received by a moving airborne station. Several inventions that use the Doppler principle for groundspeed measurement are found in U.S. Patent Classification 343-112.
Present groundspeed systems are designed for enroute operation, when groundspeed does not change significantly over short periods of time. However, during an approach to landing atmospheric changes may necessitate faster control response which requires more timely groundspeed information. Some present systems that operate on the Doppler principle normally transmit signals from the aircraft to the ground at various angles, and compare the frequencies of the transmitted and reflected signals. The difference between these frequencies is a measure of the velocity component of the aircraft's motion relative to the reflection regions, and this information with the system geometry is processed to obtain groundspeed. Another conventional technique is to measure the distance between the aircraft and ground station with distance measuring equipment (DME) that measures the time between the transmission of an interrogator pulse by the aircraft and the reception of the reply pulse from the ground station transponder. The groundspeed is related to the differnce in distance between successive measurements. These systems normally cannot meet the specific need for precision groundspeed measurements by a conventional aircraft during a final approach and landing, or by a helicopter during an approach to hover, particularly when atmospheric anomalies are encountered. Inertial navigation systems can provide the required precision, but the cost is prohibitive for most aircraft, in particular general aviation aircraft. This invention accordingly fills a need to provide this precision measurement with considerably more cost effective avionics.